A broad variety of medical infusion clamps are known in the prior art. Typically such clamps operate by clamping down on the exterior surface of an infusion tube, thereby partially constricting the flow path. Such devices are generally used in conjunction with a drip chamber, allowing visual monitoring of the actual rate of flow. Prior art devices of this general type are disclosed in U.S. Pat. Nos. 2,908,476 and 3,960,149. While such a clamp has the advantage of low cost and simplicity of operation, the use of a tube clamp to regulate flow is complicated by the fact that constriction of a tube results in cold flow of the displaced tube wall and consequent changes in the flow path shape and thus in the flow rate over the minutes or hours following clamp adjustment. In use, hospital personnel must therefore periodically monitor such flow clamps and re-set the clamp to account for the flow rate "drift." Such drift may amount to as much as 50% of the selected flow rate over an interval of 20 minutes or less. An additional limitation of such clamps is that tubing, once clamped, does not assume its original shape upon release. This lack of "memory" has as a consequence that when a tube is initially overclamped one cannot simply release the clamp and reset it to a proper value. Instead irregular flow rates will persist for some time as the tubing slowly deconstricts toward its initial shape. Each of these factors requires frequent monitoring and adjustment to attain an acceptable degree of precision in flow rate, thus placing demands upon hospital personnel. These demands can be reduced by using an automated flow adjusting device, such as the device of U.S. Pat. No. 4,137,940; however such a device is significantly more expensive than a simple plastic flow clamp. A problem thus exists as to how to construct a simple flow clamp capable of being manually set to a stable flow rate and requiring minimal human monitoring. A problem also exists as to how to construct such a flow clamp adaptable to, but not requiring, automatic control means and having the structural simplicity and ease of operation of a conventional manually operated valve. A further problem exists as to the design of such a clamp capable of stable control over a broad range of desired flow rates without such range or stability being impaired by variations the clamp elements within normal manufacturing tolerances.